Structure
Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.
Measured values
Every bar shows where radium sits among all 118 elements for that property.
Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.
Size, to scale
Radius 221 pm — that is 0.221 nm, so about 2262 million of them side by side would span a millimetre.
Thermal range
Radium is liquid over a 777 K window, from 1233 K to 2010 K.
The story
The element that glowed in the dark on watch dials, and killed the women who painted them.
Radium was discovered in 1898 by Marie and Pierre Curie in Paris, along with polonium, while investigating the mysterious radioactivity of pitchblende ore. Marie Curie noticed that pitchblende was more radioactive than pure uranium, suggesting the presence of unknown elements. Through painstaking chemical separation processes, the Curies isolated fractions containing intense radioactivity, eventually identifying two new elements they named polonium and radium.
The Curies processed literally tons of pitchblende residue in a converted shed, using primitive equipment and no safety protection. Marie Curie spent four years refining radium, finally isolating one-tenth of a gram of pure radium chloride in 1902. The work required processing over 8 tons of pitchblende to obtain this tiny sample, demonstrating radiums extreme rarity and the incredible dedication required for its discovery.
The Curies discovered that radium glowed in the dark, emitted heat continuously, and caused surrounding materials to become radioactive. These unprecedented properties revolutionized understanding of atomic structure and energy. Pierre Curie famously carried a vial of radium in his pocket, noting the burns it caused on his skin, unwittingly demonstrating both its power and danger.
The discovery of radium earned Marie and Pierre Curie the 1903 Nobel Prize in Physics (shared with Henri Becquerel for radioactivity research) and Marie Curie a second Nobel Prize in Chemistry in 1911 for isolating pure radium and determining its atomic weight. Marie Curie became the first person to win Nobel Prizes in two different scientific fields, largely based on her radium research.
Applications
Radium-226 historically played a crucial role in cancer treatment through brachytherapy (internal radiation therapy). From the 1900s through the 1970s, radium needles and applicators were inserted directly into tumors or body cavities to deliver high-dose radiation to cancer cells. While largely replaced by safer isotopes like cesium-137 and iridium-192, radium established the foundations of modern radiation oncology and saved countless lives when few other cancer treatments existed.
Radium serves as a primary standard for radioactivity measurements and instrument calibration. The original definition of the curie (unit of radioactivity) was based on the decay rate of one gram of radium-226. Today, precisely measured radium sources help calibrate radiation detection equipment, validate measurement techniques, and provide reference standards for national laboratories worldwide. Radium also contributes to nuclear physics research studying alpha decay processes.
Before its health risks were fully understood, radium had numerous industrial applications. Self-luminous paint containing radium was used on watch dials, instrument panels, and aircraft gauges from 1917 to the 1960s. These applications provided reliable illumination without external power sources, crucial for military and aviation applications during both World Wars. Radium was also used in lightning rods and as an additive in certain specialty steels.
Sealed radium sources continue to serve important educational roles in training radiation safety professionals, health physicists, and nuclear engineers. These controlled sources help students understand radiation detection principles, shielding calculations, and contamination control procedures. Modern educational applications use minimal amounts in secure devices designed to prevent exposure while providing valuable learning experiences.
Most historical uses of radium have been discontinued due to severe health and environmental risks. Consumer products containing radium, including luminous paints, health tonics, and cosmetics, were banned in most countries by the 1970s. The tragic stories of radium dial painters who developed "radium jaw" and other fatal illnesses led to strict regulations and the development of modern radiation protection standards.
Today, radium use is restricted to highly controlled applications including certain medical research, instrument calibration, and nuclear physics experiments. Ra-223 (Xofigo) represents a modern medical application - an FDA-approved radiopharmaceutical for treating bone metastases in prostate cancer patients. This alpha-emitting isotope targets bone tissue specifically, delivering radiation directly to cancer sites while minimizing whole-body exposure.
National laboratories and research institutions maintain radium sources for calibrating radiation measurement equipment and studying nuclear decay processes. These applications require strict licensing, specialized facilities, and comprehensive safety protocols. Such uses are essential for maintaining the accuracy of radiation measurements used in medicine, nuclear power, and environmental monitoring.
Radium isotopes serve as natural tracers for studying groundwater movement, ocean circulation, and sediment transport. Scientists measure radium concentrations to track water masses, estimate groundwater discharge rates, and understand biogeochemical processes in aquatic systems. These applications help manage water resources and monitor environmental changes without requiring artificial radium addition.
Where it comes from
9.00e-7 mg/kg of Earth's crust · more abundant than 28% of elements
Radium occurs naturally as an intermediate product in the uranium-238 decay chain, forming when radon-222 decays. With a half-life of 1,600 years, radium-226 accumulates in uranium-bearing ores and rocks, reaching equilibrium concentrations determined by the balance between formation and decay. Significant radium deposits occur in uranium-rich areas including the Colorado Plateau, Athabasca Basin in Canada, and various locations in Australia, Kazakhstan, and Africa.
Radium dissolves readily in groundwater, particularly in areas with uranium-bearing bedrock or sediments. High radium concentrations in drinking water supplies affect thousands of communities worldwide, especially those using deep wells in glacial aquifers or areas with phosphate deposits. The EPA sets maximum contaminant levels for combined radium-226 and radium-228 at 5 pCi/L in public water supplies.
Naturally Occurring Radioactive Materials (NORM) containing radium accumulate in oil and gas production equipment, phosphate mining operations, and coal ash. Radium concentrates in brine and scale deposits in oil field pipes and tanks, creating significant disposal challenges for the petroleum industry. Phosphate fertilizers often contain elevated radium levels, contributing to agricultural radiation exposure.
Certain building materials, including concrete made with phosphate slag or fly ash, can contain elevated radium levels. Radium in building materials contributes to indoor radon concentrations as Ra-226 decays to produce radon gas. Some natural stones used in construction, particularly granite varieties, may contain significant radium concentrations that require monitoring and potential mitigation.
Handling
Radium is radioactive. It has no stable isotope — every nucleus decays. Handling requires appropriate shielding and licensing.
Radium is extremely dangerous due to its intense radioactivity and biological behavior. When ingested or inhaled, radium accumulates in bones, where it continues emitting alpha radiation for decades. This internal exposure causes bone cancer, leukemia, and severe anemia. The tragic deaths of radium dial painters in the early 1900s, who developed "radium jaw" and bone cancers from licking radium-painted brushes, led to modern radiation protection standards.
All radium work requires specialized facilities with appropriate containment, ventilation, and monitoring systems. Personnel must wear full protective equipment including respirators to prevent inhalation of radium dust or radon gas produced by decay. Radium sources must be stored in secure, shielded containers and handled only with remote tools. Regular bioassay monitoring is required for anyone potentially exposed to radium.
Radium contamination of drinking water supplies poses ongoing public health challenges. Communities with elevated radium levels require water treatment systems or alternative supplies. Former radium processing sites, including watch dial painting facilities and medical supply manufacturers, remain contaminated decades after closure, requiring expensive cleanup efforts and long-term monitoring.
Radium spills or accidents require immediate evacuation and specialized cleanup procedures. Contaminated areas may remain hazardous for thousands of years due to radiums 1,600-year half-life. Emergency responders must be trained in radiation detection and have appropriate equipment to assess contamination levels. Medical treatment for radium exposure focuses on preventing absorption and accelerating elimination from the body using chelation therapy.
Quick answers
Radium (symbol Ra) is element 88 on the periodic table, a alkaline earth metal in period 7, group 2. The element that glowed in the dark on watch dials, and killed the women who painted them. At room temperature it is a solid, and it is radioactive.
[Rn] 7s², giving 7 occupied shells holding 2, 8, 18, 32, 18, 8, 2 electrons respectively.Radium melts at 1233 K (959.9 °C) and boils at 2010 K (1736.9 °C).
Radium has no stable isotope, so it has no standard atomic weight. The figure quoted, 226, is the mass number of its longest-lived known isotope.
Radium has a density of 5.5 g/cm³. Water is 1.0 g/cm³, so a block of radium is about 5.5× heavier.
Radium has a Pauling electronegativity of 0.9. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). A value this low means it readily gives its outer electrons away, forming positive ions.
Radium was discovered in 1898 by Marie & Pierre Curie. It is named after latin radius, "ray".
Radium makes up about 9.00e-7 mg/kg of the Earth's crust — genuinely rare, which is why it is expensive.
Yes. Radium has no stable isotope — every one of its nuclei decays. Trace amounts occur naturally as decay products of heavier elements.